Primary production of particulate protein amino acids : algal protein metabolism and its relationship to the composition of particulate organic matter

Abstract

The biochemical and physiological bases underlying primary production of particulate protein amino acids were investigated in an effort to understand the relationship between algal protein metabolism and particulate organic matter composition. In order to examine biochemical processes associated with conversion of inorganic carbon and nitrogen into protein, the effects of NH+4 limitation on free amino acid and protein composition and incorporation of inorganic 14C were studied in the marine chlorophyte, Nannochloris sp. (clone GSB Nanna). Free amino acid metabolism was sensitive to changes in steady state NH+4-limited growth rates. Reduced carbon and nitrogen flux into protein resulting from nitrogen limitation of growth was associated with reductions in proportions of cellular carbon and nitrogen in the intracellular free amino acid (IFAA) pool. Growth rate-dependent changes IFAA pool composition reflected changes in rate limiting steps which were intermediate between assimilation of inorganic nitrogen and the incorporation of nitrogen into macromolecules. The proportion of cellular carbon in both protein and pools of free amino acids decreased with decreasing growth rate. Distributions of incorporated inorganic 14C among free amino acids and protein provided qualitative descriptions of growth related compositional variations. Saturation rates of (IFAA) carbon with dissolved inorganic 14C did not significantly change as growth rates decreased. In contrast, saturation rates of free glutamate, glycine + alanine and valine did decrease with growth rate. At low growth rate, specific activities of the newly assimilated glutamate, valine, and glycine + alanine in protein were higher than specific activities of their corresponding free amino acid pools. This was likely a consequence of metabolic segregation and more rapid saturation of protein precursor pools. Enrichment of NH+4-limited steady state cultures of Nannochloris sp. with NH+4 led to a dramatic time dependent increase in free glutamine concentrations accompanied by differential increases in other free amino acids. Patterns of isotope incorporation into the free amino acids reflected real changes in concentrations. Increases were associated with the diversion of photosynthetically fixed carbon from lipophilic material towards amino acid biosynthesls, and signalled the onset of increased protein synthesis. Preliminary investigations of two other marine phytoplankton species, Dunaliella tertiolecta (clone Dun) and Thalassiosira weissflogii (clone Actin), suggested that the nature and timing of the biochemical response to NH+4 enrichment is different among different species. At high light intensity, increased NH+4 limitation of Nannochloris sp. was associated with decreases in cellular protein, protein to carbon ratios, and protein to chlorophyll a ratios. At low light intensities, cellular protein and protein to carbon ratios did not decrease with increasing NH+4 limitation. Chlorophyll a to protein ratios were generally higher at low light intensity and decreased with increasing NH+4 limitation, suggesting that nitrogen limitation suppressed low light enhancement of chlorophyll a production. Observed incorporation rates of inorganic 14C exceeded predicted rates for glycine and alanine in protein under combined conditions of light and nitrogen limitation, an indication that protein turnover in excess of net synthesis was important under these conditions. The characteristics of protein composition and incorporation of inorganic 14C were examined in steady state NH+4-limited cultures of three other species of marine phytoplankton. The species studied included Chaetoceros simplex (clone BBSM). Chattonella luteus (clone Olisth), and Chroomonas salina (clone 3C). Saturation of protein precursors was different for different species and for different protein amino acids. Nannochloris sp. displayed the most rapid and complete equilibration, while C. simplex exhibited relatively slow or incomplete saturation and high sensitivity to nitrogen depletion. Intermediate patterns were observed for the other species. For all species examined, protein glycine and alanine demonstrated relatively rapid and complete equilibration of precursor pools, and were least sensitive to nitrogen depletion. With the knowledge that these selected amino acids consistently demonstrated relatively rapid and complete equilibration of precursor pools with 14C-inorganlc carbon in several taxonomically distinct marine algae, primary production of particulate protein amino acids and its relationship to the composition of particulate organic matter was investigated in the epilimnion of a semi-enclosed marine basin, Salt Pond, MA. Studies were conducted previous to and throughout a major bloom of Olisthodiscus magnus. Before the bloom, the ratio of particulate protein amino acid (PPAA) production to particulate organic carbon (POC) production was not significantly different from the relative proportions of PPAA and POC in the particulate organic matter. Comparisons between estimated production and observed concentration changes indicated residence times of POC and PPAA were similar (10 - 20 days). Thus with respect to POC and PPAA, particulate organic matter composition reflected the composition of organic matter being produced by photoautotrophs. During the bloom decline, the PPAA/POC production ratio, a reflection of the activities of the metabolically active algal population, was significantly less than the PPAA/POC ratio of the particulate organic matter. This discrepancy can be attributed either to increased turnover of protein associated with the low inorganic nitrogen concentrations and in situ light intensities, or selective removal of PPAA carbon by secondary transformational processes.